The visual value of the control.
When the control is inactive, the value is false and this animation has
the value 0.0. When the control is active, the value is true and this
animation has the value 1.0. When the control is changing from inactive
to active (or vice versa), value is the target value and this animation
gradually updates from 0.0 to 1.0 (or vice versa).
Source
CurvedAnimation get position => _position;